They looked decent in the thumbnails, but when I gave them a closer look, I saw a bunch of JPEG or 'compression' artifacts.

Compression artifacts are a noticeable lowering/distortion of quality. For instance, if I have a very heavy graphics
scan that's around 4mb and I want to make the file size lighter, I would compress it so it'll be 1MB, or lower. If I do
it just right, the image is the same dimension, but smaller in size - and has the same sharp quality. If it's compressed
too much, artifacts appear and distort the quality, making it undesirable for many.

Here's a picture example of what I mean: [here] (the left
image is low in quality, but the right is how crisp it should look.)